There is an input circuit detects the on/off state of a switch device used by a user to operate a headlamp or a turn signal of an automobile or a motorcycle. To prevent erroneous detection of the on/off state of the switch device caused by a leak current when the switch device is wetted with water, the conventional input circuit is designed to pass the leak current to the ground.
In particular, the switch device for a headlamp or a turn signal of a motorcycle is more likely to be wetted with water, and the leak current tends to be greater, because of the structure of the motorcycle. To prevent erroneous detection and erroneous lighting caused by the leak current, the conventional input circuit has elaborate wiring designed for passing the leak current to the lamp, for example.
As conventional techniques using an LED lamp as a light, there are a technique that involves a resistor connected in parallel with the LED (see Japanese Patent Laid-Open No. 2003-63305, for example), a technique that involves detecting the on/off state of the switch device while switching (see Japanese Patent Laid-Open No. 2013-33610), a technique that involves a constant current circuit (see Japanese Patent Laid-Open No. 2014-40142), and a technique that involves using a magnet and a Hall device (see National Publication of International Patent Application No. 2014-531715, for example).